Duo Mobile

Pros

Works anywhere Google Authenticator is supported.
Duo Push feature allows two-factor integration with a single tap for LastPass and a few other sites.
Supports iOS, Android, BlackBerry, Windows Phone.

Cons

Can't sync configuration between devices.
Setting up Duo Push for LastPass proved surprisingly complicated in our testing.
Very few sites support Duo Push for consumers.

Bottom Line

The free Duo Mobile can supply two-factor authentication on any site that supports Google Authenticator.
Its Duo Push feature offers authentication with a single tap, but only after you work through its complex initial setup.

Two-factor authentication is way more secure than just relying on a password, but if it isn't super-easy people won't use it. With the FIDO Alliance's announcement that the U2F (Universal Two-Factor) specification now supports Bluetooth and NFC, it just got easier.